Power stabilization in an optical communication system

ABSTRACT

The invention concerns a method and apparatus for determining noise power caused by amplified spontaneous emission in a signal output by an optical amplifier ( 100 ). The amplifier is connected downstream of further optical amplifiers or components that generates noise power due to amplified spontaneous emission (ASE). The amplifier is connected to a control channel and receives via this channel information about the proportion of ASE power contained in the amplifier input signal. The amplifier further includes a control unit for calculating the ASE power generated in the amplifier, for example using a predetermined relationship between the gain of an amplifier and the generated amplified spontaneous emission (ASE). The ASE power in the amplifier output signal is then determined as the sum of the generated ASE power and the power resulting from propagated ASE. The determined target output power can be used to stabilise the amplifier output.

FIELD OF THE INVENTION

The invention is broadly directed to optical transmission systems thatutilise optical amplifiers, and specifically to wavelength divisionmultiplexed (WDM) systems. The invention has particular relevance to thecontrol of output power of optical amplifiers in the presence ofamplified spontaneous emission (ASE).

BACKGROUND ART

In any optical network it is important to maintain correct power levelsfor all traffic channels. This is generally achieved by monitoring theoutput power of optical amplifiers using a broadband optical detector,such as a photodetector. The monitored output power is then utilised ina feedback loop to adjust the amplifier gain so that the desired outputis produced. The amplifier output power P_(out) is then compared with adesired, or target, signal power P_(out) _(—) _(target). This targetpower P_(out) _(—) _(target) is the total power of all channelstransmitted in the optical fibre. It can thus be expressed as

P _(out) _(—) _(target) =N _(ch) ×P _(ch) _(—) _(target)

where N_(ch) is the number of channels carried in the WDM link.

However, a characteristic of optical amplifiers, whether they are of theactive fibre, semiconductor or solid state type, is amplifiedspontaneous emission (ASE) which manifests itself as a broadband signalat the amplifier output. For high input signal powers, the measurableASE power at the output is negligible. However, at low signal powers,for example powers lower than about −20 dBm, the power due ASE is asignificant proportion of the total measured output power. If the signaloutput power is corrected by adjusting the amplifier gain on the basisof the target power P_(out) _(—) _(target), the resultant channel outputpower will inevitably be lower than required.

It is known to utilise a narrow band detector at the output of theamplifier to measure the signal power at a limited range of wavelengths.This effectively filters out the ASE so that the monitored signal is afaithful copy of the output traffic power. Such a solution may be usedin systems using a single carrier wavelength, such as time domainmultiplexed (TDM) systems, however, it is not so effective for WDMsystems where a large number of different wavelengths are used. Narrowband detection may be employed for one of the signal wavelengths presentin the WDM system but is problematic for two reasons. Firstly, thesystem becomes inflexible, since the monitored signal must be routedthrough all the optical amplifiers in the network. Secondly, the systemis inherently frail because any fault occurring in the monitored channelwill result in the collapse of the whole network.

Co-pending European patent application No. 99118310.4 suggests anumerical method for determining the generated ASE power perceived atthe output of an optical amplifier. In this method all input power istreated as traffic signal power. Thus while the method is effective fordetermining how much ASE power is added to the total power output by anoptical amplifier, it does not allow stabilisation of the output trafficpower in the presence of propagated ASE power, i.e. accumulated ASEpower generated by upstream optical amplifiers and contained in theinput signal.

SUMMARY OF INVENTION

It is an object of the present invention to provide an optical amplifierarrangement and a method which enables the traffic signal power in anoutput signal to be reliably monitored.

It is a further object of the present invention to provide an opticalamplifier arrangement and a method with which the output power can bereliably stabilised.

It is yet a further object of the present invention to provide anarrangement and method for an optical communications link comprisingmultiple optical amplifiers with which the traffic power may be reliablymonitored and/or stabilised.

In accordance with the present invention, an optical amplifier receivesinformation concerning the amount of ASE noise power contained in theamplifier input. This information may be transmitted from an upstreamamplifier, or other device that generates ASE, of from a networkcontroller located at one or both ends of the link or at one or morenodes. The amplifier is further provided with a control unit fordetermining the ASE power generated within the amplifier. This ispreferably achieved by using a stored relationship between generated ASEpower and gain of the amplifier. The total ASE power at the output is acombination of the generated ASE power and the amplified received ASEpower. The total ASE power is communicated to any optical amplifierarranged immediately downstream as a fraction of the total output power,so that the same calculation may be made. Once the amount of ASE noisepower is known, the actual traffic power at the amplifier output can bededuced, and the amplifier gain may be adjusted to stabilise this outputpower.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will becomeapparent from the following description of the preferred embodimentsthat are given by way of example with reference to the accompanyingdrawings. In the figures:

FIG. 1 schematically depicts an optical communications link withmultiple amplifier stages;

FIG. 2 schematically depicts a single amplifier stage in accordance withthe present invention; and

FIG. 3 is a flow diagram illustrating a gain stabilising procedure inaccordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a uni-directional link of a WDMcommunications network. In this link an optical fibre 10 is used tocarry multiple traffic channels. Amplifier stages 20, three of which areshown in FIG. 1 for illustration and are denoted by A, B and C, areconnected at intervals along the traffic fibre 10 for relaying trafficsignals and maintaining the signal power at a desired level throughoutthe link Amplifier A is arranged in a network node 40 and serves as apower amplifier connected to an input port 41 including a multiplexer(not shown). The amplifier B is a line amplifier and amplifier C isshown arranged in a separate network Lode 50 where it serves as apreamplifier connected to an output port 51 that includes ademultiplexer (not shown).

An optical supervisory channel (OSC) 30 for relaying control messages toand from the amplifiers 20 is further connected to all amplifier stages20. The optical supervisory channel 30 preferably communicates withcontrol circuitry (not shown) belonging to a network management systemat one or both ends of the link, or at one or more nodes within thenetwork. While this supervisory channel 30 is schematically illustratedas physically separate from the optical fibre 10, it will be understoodthat the channel 30 may equally well be carried on a separate optical orelectrical cable or be carried by the fibre 10 together with the trafficdata.

It is assumed that no optical amplifier stage is connected upstream ofamplifier A. Thus power due to amplified spontaneous emission power willnot be present in the input signal to this amplifier 20. The amplifierstage A 20 itself will generate amplified spontaneous emission so that aproportion of the output power emitted by this amplifier A 20 will bedue to ASE.

In accordance with the method disclosed in co-pending patent applicationNo. 99118310.4, which is incorporated herein by reference, the desiredoutput power of this first optical amplifier stage 20 A includes the sumof desired output powers of all the traffic channels and the power dueto ASE. The desired output power thus follows the relation:

P _(out) _(—) _(target) =N _(ch) ×P _(ch) _(—) _(target) +P _(ASE)(G)  Equ. 1

Where P_(out) _(—) _(target) is the total desired output power of theoptical amplifier, N_(ch) is the number of traffic channels passedthrough the optical amplifier, P_(ch) _(—) _(target) is the desiredoutput power for each individual channel and P_(ASE)(G) is the ASE powergenerated by the optical amplifier at a specific gain G.

The method described in co-pending European patent application No.99118310.4 utilises the observation that the ASE power generated by anoptical amplifier is dependent on the gain of the amplifier to develop anumerical method and arrangement enabling the estimation of the ASEpower generated by an optical amplifier at any gain. The method involvesdetermining a model of the relation between generated ASE power andgain.

A simple model uses a linear relation between ASE power and amplifiergain, while a more complex model requires multiple measurements of ASEpower at different gains. The ASE power generated at any one gain isdetermined on production of the amplifier using a test signal of narrowoptical spectral width and high optical signal to noise ratio. A narrowband filter tuned to the test signal wavelength is connected to theamplifier output The test signal is passed through the amplifier and thepower at the output of the narrow band filter is compared with the powerat the output of a broadband filter that is likewise connected to theamplifier output. The narrow band signal power is equivalent to the testsignal power. The broadband signal power includes the test signal powerand any power due to ASE.

Each subsequent amplifier stage 20 connected in the link will generateadditional ASE. However each stage will also receive and amplifyaccumulated ASE power generated by upstream amplifiers. In the case ofthese downstream amplifier stages B, C 20, therefore, the desired outputpower is equivalent to the total desired output power of all trafficchannels plus the total ASE power. The total ASE power is equal to theASE power generated in the amplifier itself and the propagated ASE powergenerated by upstream amplifiers, or possibly by other components thatcontribute to the ASE power. This is expressed in the equation below,where P_(ASE TOTAL) (G) is the total ASE power output at the amplifiergain G. Thus

P _(out) _(—) _(target) =N _(ch) ×P _(ch) _(—) _(target) +P _(ASE TOTAL)(G)  Equ. 2

and

P _(ASE TOTAL) (G)=P _(ASE) (G)+P _(ASE prop)(G)  Equ. 3

where P_(ASE) (G) is the ASE power generated at the amplifier stage 20and P_(ASE prop)(G) is the power at the amplifier output due topropagated ASE.

As discussed above with reference to co-pending European patentapplication No. 99118310.4, it is possible to determine the ASE powergenerated at an amplifier to a good approximation. In accordance withthe present invention and as described below, each amplifier stage 20 isfurther capable of determining the amount of ASE power at the amplifieroutput that can be attributed to propagated ASE. Accordingly, with bothvalues of ASE power making up the total ASE power, P_(ASE TOTAL) (G),the gain of the amplifier can be adjusted to give the correct outputpower P_(out) _(—) _(target) to obtain the desired power for eachtraffic channel P_(ch) _(—) _(target) in accordance with Equ. 2 above.

The propagated ASE power is determined as follows. The first amplifier20 in the amplifier chain, in this case amplifier A, determines thegenerated ASE power P_(ASE) _(—) _(A) (G) contained in the total outputpower at the gain utilised, for example using the method and possiblythe arrangement described in co-pending European patent application No.99118310.4. The gain, and therefore the generated ASE power, ispreferably set to provide the desired target power for all trafficchannels in accordance with Equ. 1. The value of ASE power is thenexpressed as a proportion R_(ACC ASE A) of the total output powerP_(OUT A) in accordance with the following expression:

R _(ACC ASE A) =P _(ASE) _(—) _(A)(G)/P _(OUT A)  Equ. 4

Since no ASE power is propagated through amplifier A, the total ASEpower contained in the amplifier output power is due to the generatedASE power P_(ASE) _(—) _(A) (G)

The ASE proportional value R_(ACC ASE A) is then transmitted to thefollowing amplifier stage B in the link. This amplifier B utilises thevalue R_(ACC ASE A) to calculate the total proportion of ASE power,P_(ASE TOT B), contained in the amplifier output power in accordancewith the following expression

 P _(ASE TOT B) =P _(ASE) _(—) _(B)(G _(B))+R_(ACC ASE A) ×P _(IN B) ×G_(B)  Equ. 5

where P_(ASE) _(—) _(B) is the ASE power generated by amplifier B,P_(IN B) is the total input power to amplifier B and G_(B) is the gainof amplifier B. From equations 2 and 5, and with knowledge of the totalinput power, P_(IN B), the required gain, G_(B), for obtaining thedesired channel power, P_(ch) _(—) _(target) can be determined and theamplifier pump power adjusted accordingly.

In addition, amplifier B determines the proportion R_(ACC ASE B) of theamplifier output power P_(OUT B) that is attributable to the accumulatedASE power, P_(ASE TOT B) in accordance with the relation expressed inEqu. 4. This proportional value, R_(ACC ASE B), is then transmitted tothe next amplifier connected in the link, in this case amplifier C.Amplifier C then performs the same steps as amplifier B to determine thetotal ASE power contained in the output, calculate the required gainG_(C) and the accumulated ASE proportion value R_(ACC ASE C) andtransmit this propagated ASE proportion value to a following amplifier,and/or possibly to the network management system.

The communication of the accumulated ASE proportion values R_(ACC ASE)is preferably accomplished using the optical supervisory channel 30, orother suitable control channel. As discussed above the control channel30 may be carried by the optical traffic fibre 10.

The first amplifier stage 20 in an amplifier chain may be configured oninstallation such that it determines only generated ASE power.Alternatively, it may receive information from the management system viathe optical supervisory channel 30 or other suitable arrangement forrelaying control messages. This information may specify the amplifierconfiguration, i.e. that it is the first amplifier in a chain ofamplifiers, thus preventing the amplifier from calculating propagatedASE power. The information may instead take a similar format to theproportional ASE figure, and merely specify this proportion is zero.

FIG. 2 schematically depicts an optical amplifier arrangement fordetermining the total ASE power carried over an optical link In thefigure, an optical amplifier 20 is connected as a line amplifier in anoptical fibre 10. A first optical coupler 60 is spliced to the fibre 10at an input side of the amplifier 20 while a second optical coupler 70is connected to the fibre 10 at the output of the amplifier 20. In theillustrated example the amplifier 20 is an active fibre amplifier thatis driven in the conventional manner by one or several pump lasers 21.However, the amplifier 20 may equally be any class of optical amplifierthat generates amplified spontaneous emission. For example it maycomprise, but is not limited to, a rare earth doped fibre amplifier, anundoped fibre amplifier such as a Raman or Brillouin amplifier or asemiconductor laser amplifier. It will be understood, that when theamplifier 20 is a semiconductor laser amplifier, the pump laser 21 wouldbe replaced by a current or voltage supply controlling the laser.

The optical couplers 60, 70 are selected to extract a small proportionof the transmitted signal power and typically have an extraction ratioof around 1:20. These couplers may be any suitable device capable ofextracting a portion of the light signal carried in the optical fibre10. An opto-electric converter 80, which may take the form of aphotodetector, phototransistor, or any other suitable conversion device,is connected to the first optical coupler 60 and converts the extractedlight signal into an electrical signal. A further opto-electricconverter 90, similar to that connected to the amplifier input, isconnected to the output of the second coupler 70. The extracted andconverted input P_(IN) and output signals P_(OUT), which represent ameasurement of the amplifier input and output powers, respectively, arefed to a control unit 100.

The control unit 100 is further connected to the optical supervisorychannel 30 and receives via this channel the proportional R_(ACC ASE)figure representing the ratio of ASE power to traffic signal poweroutput by the immediate upstream amplifier. It is assumed that thisratio remains substantially constant over the fibre 10 connecting theamplifier 20 with the upstream amplifier. This figure R_(ACC ASE)therefore represents the fraction of ASE power input into the amplifier20. The unit 100 is also coupled to the amplifier pump 21 forcontrolling the amplifier gain by modifying the current or voltagesignal controlling the pump laser. The control unit 100 preferablycontains software controlled processing means, such as a microprocessor,microcomputer or the like, together with associated memory devices andpossibly peripheral devices enabling monitoring of the amplifieroperation. In addition to receiving and transmitting the proportionalASE figure, R_(ACC ASE), the control unit 100 may also receive andtransmit other information relative to the amplifier condition to thenetwork management system via the optical supervisory channel 30.

The function performed by this control module is illustrated in a flowchart depicted in FIG. 3. This flow chart starts with step 101, in whichthe traffic power input into the amplifier 20 is determined using theproportional ASE figure R_(ACC ASE) received from an upstream amplifierand the measured input power P_(IN). The traffic input power calculatedin this step effectively ignores other noise components, and assumesthat all power not due to ASE is due to the N traffic channels. In step102 the input traffic power is used to determine the amplifier gainrequired to obtain the sum of the target channel powers N_(ch)P_(ch)_(—) _(target). The required or desired traffic channel power P_(ch)_(—) _(target) may be a fixed quantity stored in the control unit 100 orin a component that is readily accessible to the control unit 100.Alternatively, the target power may be communicated to the amplifier,for example by the network management system, via the opticalsupervisory channel 30 or other control channel. The calculated value ofgain G is then used in step 103 to determine the ASE power generated bythe amplifier.

In accordance with the method and arrangement described in co-pendingEuropean Patent application No. 99118310.4, the ASE power can bedetermined either by calculation using a stored model of therelationship between gain and ASE power or by referring to a storedlookup table containing values of ASE power for multiple gain figures.The stored model or lookup table is preferably established onmanufacture of the amplifier and may be specific to the amplifier or toa class of amplifiers.

In step 104, the total ASE power in the amplifier output signal isdetermined by summing the generated ASE power with the propagated ASEpower in accordance with Equ. 5. The total required or target outputpower P_(out) _(—) _(target) is then calculated in step 105. In step106, the real, measured output power of the amplifier P_(OUT) iscompared with the calculated target output power P_(out) _(—) _(target).If these are not equal the pump power of the amplifier pump 21 isadjusted until they are substantially equal. Finally, in step 108, theproportion of ASE power R_(ACC ASE) contained in the amplifier outputsignal is calculated. This value is then ready for transmission to theoptical amplifier located directly downstream of amplifier 20 for use ingain stabilisation. Preferably, this value is transmitted together withinformation identifying the sending amplifier, and/or the receivingamplifier. This proportional ASE figure may also be utilised by thenetwork management system to monitor the operation of the network andalso of individual amplifiers.

It will be understood that the amplifier gain need not be adjusted.Instead the network may be designed to tolerate a certain quantifiabledrop in power. The arrangement and method described with reference toFIGS. 2 and 3 can then be used to provide information about the power ofthe traffic channels at any point along the optical communications link.In this case the real gain of the amplifier is calculated utilising themeasured input and output powers.

What is claimed is:
 1. An optical amplifier arrangement, including anoptical amplifier disposed to receive optical input signals and emitamplified optical output signals, means (60, 80) for measuring the inputpower to the amplifier (20), characterised by means (30, 100) forreceiving data representing the amount of ASE noise power in an inputsignal received by the amplifier (20), and means (100) responsive tosaid measured input power and said received data for determining theamount of ASE power generated in the amplifier and for calculating thetotal ASE noise power at the amplifier output.
 2. An arrangement asclaimed in claim 1, further characterised by means (100, 30) fordetermining the proportion of ASE noise power (R_(ACC ASE)) contained inthe amplifier output power and for transmitting a control messagecontaining this determined proportion.
 3. An arrangement as claimed inclaims 1 or 2, further characterised by means (70, 90) for measuring theoutput power of the amplifier (20) and means (100) for determining theamplifier gain.
 4. An arrangement as claimed in claim 3, characterisedby means (100) for determining a required output power of the amplifier(20) for a predetermined traffic output power, means (70, 90) formeasuring the output power of the amplifier (20), and means (100, 21)for adjusting the amplifier gain when the measured output power is notsubstantially equal to the required output power.
 5. An opticalamplifier arrangement including an optical amplifier (20) coupled to onan optical transmission line for receiving optical input signals andemitting amplified optical output signals, means (60, 80) for measuringinput power to the amplifier (20), means (70, 90) for measuring poweroutput from the amplifier, characterised by means (30, 100) forreceiving data representing the proportion of ASE noise power in theamplifier input, means responsive to said measured input power and saidreceived data for determining the received traffic power, means forcalculating the amplifier gain required to obtain a desired trafficoutput power, means (100) for determining the amount of ASE powergenerated at the calculated amplifier gain, means for determining theamount of amplified propagated ASE noise power contained in an outputsignal and means (100, 21) for comparing the measured amplifier outputpower with the sum of the desired traffic output power, the generatedASE power and the amplified propagated ASE noise power and for adjustingthe amplifier gain if these are not substantially equal.
 6. An opticallink including an optical fibre (10) with at least two opticalamplifiers (20) connected to said optical fibre and a control channel(30) arranged to carry control messages to and from said opticalamplifiers, said link including means (60, 80) for detecting the inputpower to each amplifier, characterised by means (100) for receivingthrough said control channel (30) information concerning the amount ofASE noise power contained in the input to each amplifier, means (100)for determining ASE noise power generated in each amplifier and means(100) responsive to said measured input power and said received data forcalculating the total ASE noise power at the output of each amplifier.7. A link as claimed in claim 6, further characterised by means (100)for calculating the fraction of ASE noise power in the power output fromeach amplifier (20) and transmitting information concerning saidcalculated fraction on said control channel (30).
 8. A link as claimedin claim 6 or 7, characterised in that said control channel (30) iscarried on a cable that is physically separate from said optical fibre(10).
 9. A method of determining the amount of noise power due toamplified spontaneous emission in a signal output by an opticalamplifier, including: measuring the input power to the amplifier,receiving information concerning the proportion of propagated ASE noisepower in a received signal, determining the amount of amplifiedpropagated ASE power contained in the amplifier output signaldetermining ASE power generated by said amplifier, summing saidgenerated ASE power and said amplified propagated ASE power to obtainthe amount of noise power in the amplifier output signal.
 10. A methodas claimed in claim 9, characterised in that said generated ASE power isdetermined by measuring the output power of the amplifier, calculatingthe gain of the amplifier and determining the generated ASE power usinga stored relation between ASE power and gain of the amplifier.
 11. Amethod as claimed in claim 9, characterised in that said generated ASEpower is determined by determining the traffic signal power in theamplifier input signal, calculating an amplifier gain required to obtaina desired traffic signal output and determining the generated ASE powerusing a stored relation between ASE power and gain of the amplifier. 12.A method as claimed in any one of claims 9 to 12, characterised bydetermining the ratio of ASE power to traffic signal power in theamplifier output signal and transmitting said ratio to at least anupstream optical amplifier.
 13. A method of stabilising the output powerof an optical amplifier in an optical communications link, including:measuring the input power to the amplifier, receiving informationconcerning the proportion of propagated ASE noise power in a receivedsignal, determining the received traffic power, determining amplifiergain for obtaining a desired traffic output power, determining ASE powergenerated at said gain, determining the amount of amplified propagatedASE noise power contained in an output signal, adjusting the realamplifier output power to be substantially equal to the sum of thedesired traffic output power, the generated ASE power and the amplifiedpropagated ASE noise power.